Such a device is disclosed in Japanese patent unexamined publication Sho 62-64299 and its U.S. Pat. No. 4,636,706 to Bowman as one of the conventional power control devices of alternators.
However, the device disclosed in the above publication is equipped with an up-down counter (a binary counter which can count up and down) which is controlled according to a result of comparison of an output voltage of an alternator with a reference voltage and a PWM circuit (pulse width modulation circuit) which includes a down-counter and generates pulse signals having pulse width changing according to the number counted by the up-down counter to supply a field current to the alternator field coil. Therefore, many digital circuit elements are required and, consequently, the digital circuit becomes complicated.
Another object of the present invention is to provide a simple digital circuit which changes the field current gradually.
Another object of the present invention is to provide means for controlling the field-current switching means to have different conducting times between increasing the field current and decreasing the field current without difficulty.
A further object of the present invention is to provide means for controlling the field current switching means to synchronize an output timing of comparison between the alternator output voltage and a reference voltage with the switching timing of the field-current switching means so that the switching means can switch on or off steadily even under an abrupt output voltage change.